blog article

Part 2 of Concept realization of an 4x4 Vehicle's Transfer case

Feb. 18, 2015

Marcel Romijn

BRACE Automotive executed a demonstrator project in cooperation with a thesis student of the Rotterdam University of Applied Sciences. Goal is to demonstrate the capabilities of BRACE in gears, bearings and housing design of a transmission. The demonstrator product is a transfer case for a 4x4 vehicle that can work together with an automated transmission. The transfer case can handle the low- and high-gearing selection and the torque distribution function between front and rear axle. Also the differential is equipped with a differential lock. Shifting between low and high gears and engaging the differential lock is controlled by actuators.

This project stage mainly focusses on the mechanical aspect of the transfer case and provides a concept of the gear train as a result. The previous blog entry showed how a first concept of the transfer case came into being by establishing the necessary functionalities and making certain design choices based on the customer’s requirements. This second blog entry will cover the calculation and simulation procedures and the realisation of the final model.

The first step in creating the geometry of the gears was to perform some initial calculations by multiplying the module and the number of teeth of each gear and calculating the center distances. By using the right number of teeth for each gear pair, the desired ratios given by the customer could be achieved.

The next step was to simulate the entire system in KISSsys software, which meant a kinematic model had to be created. KISSsys is a software program that can calculate the torque and power through a system with certain pre-defined load parameters.
With these input parameters, KISSsys can also calculate the shaft deflection, stress and bearing lifetimes for the two different power flows through the system that are required. A key result that had to be extracted from the KISSsys simulations were the shaft deflection values. These values have a strong impact on the lifetime of the gears, because shaft deflection causes a one-sided contact pattern on the gear flanks.

In order to finalize the calculations, the micro geometry of the gears also had to be defined. This meant entering the right parameters in KISSsoft to create the gears. KISSsoft software can be used to define the micro geometry of a gear pair and to calculate the expected lifetime according to a pre-defined load.
For this calculation, power, torque and speeds according to the requirements were collected with the shaft deflection values in a load spectrum. With these parameters, the lifetime and contact analysis of each gear pair was calculated.

The results showed stress at flank ends and an unequal stress distribution along the gear teeth of certain gears. Another observation was made that stress that occurred on the base of a gear flank resulted in a bending effect of the tooth causing the expected life time to drop. Regarding the results for the shafts, a stress peak could be observed in the center of the shaft and the largest displacements occurred at the shaft ends. This was all due to the deflection caused by the forces acting on the driven gears, which bends the shaft a little bit.

After analyzing, the results showed that gears with gear teeth modifications, such as a helix angle modification and lead crowning, had a much better distribution of stress along the gear flank and a better tolerance for misalignment.

The geometries that were calculated in KISSsys and KISSsoft were imported in to 3D CAD in order to create the final model. By creating three different variations, which had all different solutions for key aspects like weight reduction, axial fix of the components, lubrication and manufacturability, a convergence could be made to one final optimized model. In this model, the differential could be placed on the inside of the crown gear, resulting in a more compact design and a better alignment of the bearings. This was very beneficial for a future housing design.

The setup of all the parts is visible in this video.

On a sidetrack a short attempt at 3D printing of certain parts was made. The end goal is to have a 3D printed transfer case for display purposes. To gain more experience with the 3D printing one shaft and a couple of gears were 3D printed.

More work is still needed for optimization and the housing is not designed yet. Stay tuned for further updates.